The present invention relates generally to an input/output control system and particularly to an input/output control system in which a buffer is provided for attaining cost reduction. In particular, the invention concerns an input/output control system for controlling magnetic tape devices, wherein each of the active magnetic tape devices (i.e. the devices mounted on tape decks) is allocated with at least two buffer areas or fields, to thereby allow the preload and/or after-write operation of data to be performed asynchronously with the input or output request from a main storage.
There has been proposed a control system in which a buffer is provided in an attempt to improve the cost-performance of an input/output system including magnetic tape devices.
The input/output control system for the magnetic tape devices in which the buffer is incorporated can be classified into two categories or types. According to one type, a control device having a buffer is connected to the hitherto known magnetic tape devices. In the other type of control system, the control device incorporating the buffer is connected to the magnetic tape devices from which vacuum column is removed. The first mentioned type control system is commercially available at present. The second mentioned type control system is disclosed in Japanese Patent Application Laid-Open Nos. 41955/1984, 43766/1984. Since the vacuum column is indispensable for a high-speed start action of the magnetic tape device, the time required for the start action of the magnetic tape device provided with no vacuum column is increased remarkably. Accordingly, the start action is performed on the off-line basis as in the case of the disc device. More specifically, in the case of the disc device, the off-line operation corresponds to the seek action and the search action. In contrast, in the case of the magnetic tape device, the off-line operation is performed for rewinding the tape to a position located immediately before the read-write position.
Further, there has also been proposed a control system in which a buffer is provided for permitting the preload and/or after-write operation of plural data blocks with a view to reducing the overhead corresponding to the time taken for the start operation of the magnetic tape devices.
In conjunction with suppression of the performance degradation brought about by the removal of the vacuum column to a minimum, there arises a problem with regard to the method of the preload and the after-write. More specifically, when the amount of data to be transferred for the preload and/or the after-write operation is insufficient, the frequency at which the start action is performed will be increased, involving degraded efficiency of utilization of the data transfer path.